This invention relates to methods and apparatus for measuring the impedance of a patient's body, and more particularly, to techniques for measuring body impedance for use by a rate-responsive pacemaker.
Pacemakers are used to treat a variety of cardiac conditions. Some pacemakers simply provide pacing pulses to a patient's heart at a fixed rate. More sophisticated devices contain sensing circuitry that allows the pacemaker to monitor a patient's heartbeat signals. For example, some pacemakers can monitor a patient's atrial heartbeat signals and provide corresponding ventricular pacing pulses, which allows the patient's cardiac output to be adjusted depending on the patient's intrinsic atrial heart rate.
However, in many situations there is no reliable normal heart rhythm that can be monitored by a pacemaker. Because the cardiac need of a patient varies depending on the patient's physical activity level, rate-responsive pacemakers have been developed that provide pacing pulses at a rate based on the patient's need for cardiac output. Some rate-responsive pacemakers contain accelerometer-based activity sensors, which assess a patient's level of physical activity by measuring the patient's body movements. When the measured frequency and intensity of a patient's movements are high, the patient's heart is paced at a correspondingly high rate. Although this approach is generally satisfactory, many rate-responsive pacemakers that use activity sensors are unable to clearly differentiate between body movements due to physical activity and body movements due to external sources (e.g., body movements experienced during an automobile ride).
Other rate-responsive pacemakers use oxygen sensors to measure a patient's blood-oxygen level. Rate-responsive pacemakers that use oxygen sensors adjust the pacing rate to maintain a suitable oxygen level. However, oxygen sensors require the use of a special pacemaker lead.
Another approach that has been used to assess a patient's need for cardiac output is to attempt to determine the amount of air being inhaled by the patient. Taking breaths deeply and frequently indicates that there is a high need for cardiac output. When a patient inhales, the pressure in the chest cavity drops, which causes the impedance of the chest cavity to drop. Measuring the impedance of the chest cavity has been found to provide a good indication of the amount of air being inhaled by a patient. An advantage of monitoring the impedance of the chest cavity to assess cardiac need is that the pacemaker is unaffected by body movements due to external sources and does not require the use of special leads.
One way for the pacemaker to measure body impedance is to apply a current signal of a known magnitude and waveform across the patient's chest. The resulting voltage signal across the body can be measured by sensing circuitry. The impedance is calculated based on the known magnitude of the applied current signal and the measured magnitude of the voltage signal.
Although signals with low frequency content are suitable for measuring body impedance, they often interfere with measurements made using external equipment such as electrocardiogram (ECG) machines, which are sensitive to signals in the 1 Hz to 240 Hz range. Interference from the signal used for impedance measurements is undesirable, because it makes reading the ECG signal difficult.
What is needed therefore is a technique for measuring body impedance without interfering with external cardiac monitoring equipment such as ECG machines.